1. Field of the Invention
The present invention relates to a data storage device, and more particularly, to a structure for securing one or more disks within a disk drive, the structure exerting a clamping force which is substantially uniformly distributed around the disk.
2. Description of the Related Art
Conventional disk drives for use in work stations, personal computers, and portable computers are required to provide a large amount of data storage within a minimum physical space. In general, Winchester type disk drives operate by positioning a read/write transducing head over respective tracks on a rotating magnetic storage disk. Positioning of the head over the tracks is accomplished by an actuator coupled to control electronics, which control the positioning of the actuator and the read/write functions of the heads.
Greater demands are being placed on disk drives by (1) the use of multi-user and/or multi-tasking operating systems, (2) work stations which provide an operating environment requiring the transfer of large amounts of data to and from a hard disk and/or large numbers of disk accesses to support large application programs or multiple users, (3) the present popularity of notebook and laptop computers, and (4) the continuing trend toward higher performance microprocessors. All such systems require a hard drive having high-capacity storage capability, while occupying a minimum of space within the host computer. In order to accommodate these demands, there is a need to produce a smaller hard disk drive which at the same time has an increased storage capacity. For such applications, single drive capacities on the order of hundreds of megabytes are common.
An important determinant in the storage capacity of a disk drive is the flying height of the transducing heads above the rotating disk. In conventional Winchester-type hard drives, once the storage disk achieves a certain angular velocity after start-up of the drive, a cushion of circulating air above the surface of the disk forces the head up off the surface of the disk to thereby achieve a flying height. Having very low flying heights offers several advantages, primary among them is that flying the head very close to the disk surface allows for a high data bit density (i.e., the number of data bits per inch on a data track). The greatest data bit density would be obtained where the transducing head rides in contact with the storage disk. However, the contact of the head and head slider with the disk surface would result in damage to the head and/or disk in an unreasonably short period of time. Thus, there has been an industry wide push to decrease the height at which read/write heads are maintained over the disk surface without actually riding in contact with the disk surface. In the 1960's flying heights were commonly about 100 microinches (.mu."). At present, technological advances in read/write heads and disk drive design have allowed the reduction of flying heights to around 4.mu." in commercially viable disk drives.
When a head flies over a disk, the flying height does not remain constant, but rather tends to fluctuate slightly above and below the normal flying height. At lower flying heights, a variation in the fly height may cause the head to randomly contact the disk surface. This situation is referred to as intermittent contact. Presently, flying heights have been reduced to the point where intermittent contact with the disk surface has become an important consideration in the tribology of the head/disk interface. Repeated intermittent contact between the head and a particular location on the disk surface can cause damage to the head and/or disk, and may cause drive failure in an unreasonably short period of time.
In contemporary disk drives, such as the 21/2 and 31/2 inch form factor drives, special attention must be paid to the structure and method used to clamp the disk or disks within the drive. Due to a combination of factors, distortion in the disk caused by conventional disk clamps has become a significant problem near the inner diameter of the disk. One reason distortion has become a problem relates to the present flying heights of the read/write head over the disk. With the head flying so close to the disk, even slight distortions in the disk surface can cause repeated intermittent contact and drive failure in a short period of time. A second reason why distortion has become a problem relates to the thickness of the disk. In previous generation disk drives, such as the 51/4 inch and larger form factor drives, the disks used were relatively thick, and thus were able to withstand the clamping force of the disk clamp without significant distortion. However, in an effort to minimize the height of 31/2 inch and smaller form factor drives, the thickness of the disks used has been reduced to the point where distortion of the disk by the clamp is a potentially significant problem.
Another reason why disk clamps pose significant disk distortion problems relates to the manner in which the clamp secures a disk within the disk drive. In conventional drives, the disk is provided on a cylindrical hub which is affixed to the rotor of the spin motor. A clamp is provided on top of the hub, and has a larger radius than the hub such that an outer circumferencial portion of the clamp is in contact with the disk. A plurality of screws fit through holes around an inner circumferencial portion of the clamp, and into threaded bores in the hub. Conventionally, anywhere from three to eight screws are-used in this type of clamp configuration. In smaller disk drives which do not require a large clamping force, a clamp having a single screw through the center of the clamp may be used. When the screws are tightened, the pressure at the screws is transferred to the outer circumferencial portion of the clamp, which contacts the disk to secure the disk or disks in place. The screws must be secured under a considerable force in order to prevent any slippage, radial movement or tilting of one or more disks. Even a very slight radial movement or tilting of a disk within a drive could result in mechanical off-tracking of the head with respect to the disk, which off-tracking could result in read/write errors.
Ideally, the force exerted by the disk clamp at the circular line of contact defined between the clamp and disk should be uniform around the entire line of contact. However, in fact the concentrated force of the screws securing the clamp to the hub result in localized stresses at points around the line of contact located radially outward from the screws. These localized stresses tend to distort the disk. The stresses are greatest near the inner diameter of the disk, and tend to dissipate toward the outer diameter of the disk, so as to create distortion in the disk similar to patterns 10 on FIG. 1.
Attempts have been made in the past to reduce the localized stresses transmitted to a disk in one of two ways. The first way was to use a more rigid material. Conventionally, disk clamps have been formed from aluminum. However, by using, for example, a clamp machined out of steel, it was hoped that the higher rigidity would more uniformly distribute the stresses due to the concentrated screw force. The second way was to use a thicker disk clamp, thereby adding to the rigidity of the clamp. Upon testing, both attempts failed to significantly reduce the distortion in the disk. It is believed that these designs failed because of the uniform rigidity throughout the clamp. While the uniformly higher rigidity in the clamp was effective to a small degree in distributing stress, contact areas between the disk and clamp radially outward from the screw points still exhibited a greater contact pressure than contact areas radially outward from positions between two screws. The rigidity of a uniformly rigid clamp could be increased to the point where localized stresses at the screw points were uniformly distributed. However, such clamps are either not economically feasible or too cumbersome to fit within a disk drive.
At the inner diameter of the disk, the peak to valley distortion of the disk due to non-uniform clamping force may be as high as 10 to 20 microns. Since read/write heads are presently flying at normalized surface heights of less than approximately 4.mu.", it is clear that flying the head near the inner diameter of the disk will result in severe and repeated intermittent contact of the head with the high points of the disk, which may result in damage to the head and/or disk and drive failure in an unreasonably short period of time.